For quite some time, water-resistant paper and paperboard articles have been used as wrapping or packaging for moist or wet foods, many of which are refrigerated. For example, corrugated paperboard containers are cascade-coated or curtain-coated with paraffin wax-based compositions applied by passing the corrugated paper containers beneath a curtain of molten paraffin wax. These water-resistant containers are then used for packaging produce, meats, poultry and fish that must be refrigerated. Often the containers themselves are filled with ice in addition to the food itself. Ordinary untreated paperboard containers cannot withstand such exposure to water.
U.S. Pat. No. 4,668,536 to Goodell et al. teaches an apparatus for applying a coating of paraffin wax to sheets of corrugated paperboard by feeding the sheets through a bath of molten wax held in an elongated tank. Corrugated paper containers produced by such processes have not been considered recyclable because the wax cannot be efficiently separated from the fibers making up the paper using conventional repulping equipment.
Ecological concerns have focused on disposal of water-resistant paper and corrugated paper containers because landfills have been filling up at an alarming rate. If such containers could be recycled, this would reduce the amount of waste containers that must be placed in landfills as well as the amount of virgin timber needed to produce the paper.
U.S. Pat. No. 1,628,931 to Todd teaches a process for extracting wax from waxed paper by treating it with a solution of aqueous trisodium phosphate alone or with other alkaline materials. The wax becomes emulsified, leaves the paper fibers and rises to the top of the treating solution where it can be skimmed off. The process also serves to de-ink the papers.
An article by J. S. Michelman et al. entitled "Repulpability of coated corrugated paperboard" in the October 1991, Tappi Journal, pp. 79ff, notes that curtain coated hot melt wax compositions are considered to be non-repulpable. It proposes the use of water-based coatings in place of hot melt wax curtain coatings. The Michelman et al. article states water-based coatings provide certain advantages in that they do not contain solvents, are nonflammable and yet have the ability to provide water and grease resistance to corrugated paper containers.
Water-based coatings do provide these advantages, but their use requires a change in the type of coating equipment used to manufacture coated, water-resistant paper, paperboard and corrugated paper containers. In practice, these coatings do not have sufficient water resistance to withstand long periods of exposure to ice water such as is required for produce or meat containers. Use of water-based coatings on hydrophilic substrates such as corrugated paper produces a wet surface that must then be dried by the application of heat.
Hot melt coatings are typically solvent-free and applied at 60.degree.-95.degree. C. depending upon the type of water-resistant agent being used. Hot melt coatings immediately solidify as they cool below their melting point. Thus, they do not require a significant further drying step of the type which may be necessary with water-based coatings. An increase in the drying time or energy needed to dry the coatings is a disadvantage because it increases cost and lengthens the time needed before the paper or corrugated paper containers can be handled. In a conventional cascade or curtain coating process, corrugated paper containers are placed at one end of a cascade or curtain coating machine having a "curtain" or wall of molten wax coating that is poured continuously over the containers passing through the curtain. After coating, the containers pass through an area of the machine that has a fan to blow ambient air over the containers and reduce their temperature to the point where the hot melt coating solidifies. The containers are then simply removed from the other end of the machine and stacked for shipping. Thus, hot melt coatings have the advantage of being relatively easy to process. They do not require a change in coating equipment from the type of equipment already owned by many commercial manufacturers of coated paper and corrugated paper containers.
Therefore, there is a need for a hot melt coating for use with paper such as wax paper for food wrap, paperboard containers such as milk and juice cartons and corrugated paper containers such as those used to package produce, meats, poultry and fish that permits the paper articles to be repulped and recycled, but can be applied using conventional hot melt wax coating equipment.
Patent Cooperation Treaty Published International Patent Application No. WO 91/05107 to Michelman teaches repulpable hot melt paper coatings which are a combination of at least one wax and at least one chemical compound incorporated within the wax where the compound is capable of dispersing the wax in a substantially aqueous environment. The compound must by itself act as a dispersant for the wax or must be capable of being chemically modified so as to act as a latent dispersant for the coating in an aqueous environment. The compound may be a nonionic, anionic or cationic emulsifier, dispersant, surfactant or surface acting agent. Examples of anionic dispersants are said to include acids, esters, alcohols, sulfonates and soaps. A specific example of an anionic dispersant given is stearic acid in an amount of from about 10% to 30% by weight and it is becomes capable of emulsifying or dispersing the coating by subjecting the coating containing the stearic acid to an aqueous solution of a strong base such as sodium hydroxide or potassium hydroxide. However, the Michelman International Application does not suggest our use of polymers which contain carboxyl groups which will be described below.
A recent article from the April, 1992, Tappi Journal, Vol. 75, No. 4, pages 37 and 39 refers to work done by IL Returpapper, the largest Scandinavian wastepaper collector, and some of its owners on the repulping of waxed corrugated boxes. The article reports that their work focused on the use of self-dispersing waxes based on the addition of aluminum stearate soaps to the wax blend. The article states such aluminum soaps neither reduce the water resistance nor increase the water vapor permeability of the wax film. Under slightly alkaline conventional repulping conditions, the aluminum soaps are said to dissociate and form sodium soaps that disperse the wax away from the paper pulp.
One problem associated with prior art hot melt wax coatings is a tendency for such coatings to stress crack when the coated article is bent or formed.
U.S. Pat. No. 3,629,171 to Kremer et al. provides a hot melt wax composition having reduced stress cracking composed of a wax such as paraffin, an ethylene-vinyl acetate copolymer, a sufficient amount of a wax-compatible, noncorrosive carboxylic acid such as oleic acid, lauric acid, and pentacosanoic acid to provide the composition with a total acid number of from 5 to 200, and a solid butyl rubber. The carboxylic acid can be present in an amount of from 10-150 parts per 100 parts of wax such as paraffin. Nothing is said about the repulpability of such compositions.
U.S. Pat. No. 3,417,040 to Kremer is similar to the '171 Patent in that it requires a wax such as paraffin, an ethylene/vinyl acetate copolymer and an acid component. However, the acid component must contain at least 35% by weight acid and at least 5% by weight liquid of boiling point greater than 300.degree. F. (149.degree. C.) at 7 mm Hg pressure. The goal is a wax composition with increased melt viscosity that permits lowering the ethylene/vinyl acetate copolymer concentration in the coating. Examples of liquids are liquid organic carboxylic aids, mineral oils or a styrene, butene or propene polymer.
U.S. Pat. No. 3,001,963 to Higgins teaches hot melt wax compositions that have a reduced tendency to foam due to the presence of a mixture of a polymeric vinyl ester such as polyvinyl caproate and stearic acid. Higgins teaches that the stearic acid is preferably used in an amount that does not exceed 0.1%. Similarly, the polyvinyl caproate is used in an amount of from about 1 to 25 p.p.m. of the total composition.
U.S. Pat. No. 3,338,856 to Arabian et al. teaches blends of waxes with up to 50% of copolymers of ethylene and terminally ethylenically unsaturated organic compounds containing 3-5 carbon atoms such as alpha-olefins, vinyl acetate, and ethyl acrylate. The presence of the polymer is said to reduce solid phase separation or blooming on the surfaces of the coatings made from such blends. The coatings are used on paper and carton boards as well as fibers and cloth. Nothing is taught concerning repulpability.
U.S. Pat. No. 3,165,485 to Ilnyckyj et al. teaches sealing waxes used to coat materials such as paper for wrapping bread where the wax further includes 10-40% of a copolymer of ethylene and a diester of fumaric or maleic acid. Nothing is taught concerning repulpability.
U.S. Pat. No. 4,362,784 to Kato et al. teaches packaging materials for food products which are poured into the package in a molten state. The packaging materials are coated with a blend of (a) an ethylene/vinyl acetate polymer, (b) a copolymer of ethylene or an alpha olefin with, for example, maleic anhydride, optionally, (c) a tackifying agent such as a specific blend of (a) with an olefin/maleic anhydride resin, and, optionally, (d) 1-10% of a wax.
Other wax-based compositions that can be applied by a hot melt method as well as from a solvent solution include U.S. Pat. No. 2,599,339 to Lippincott et al. Lippincott teaches improved wax compositions that contain copolymers of ethylene-1,2-dicarboxylic acids such as maleic acid with comonomers such as ethylene, octylene and octadecene as well as other unsaturated acids such as acrylic acid or unsaturated esters such as vinyl acetate or allyl stearate. Before adding the copolymer to the wax to make the coating composition, the copolymer is modified with an alcohol or an amine such as ethyl alcohol, ethyl amine, diethylamine, octadecyl amine or dioctadecylamine. The waxes are then modified by the addition of 0.5% to 5% or more, preferably from 0.1% to 1% of the modified copolymer.
U.S. Pat. No. 2,676,934 to Butler teaches hydrocarbon wax compositions for impregnating wrapping paper that further contain from 0.1% to 30% by weight of an esterified styrene-maleic anhydride resin. The styrene-maleic anhydride polymer is completely esterified with a fatty alcohol such as decyl, dodecyl or hexadecyl alcohol. The coating is said to be more resistant to cracking and crushing.
U.S. Pat. No. 3,428,591 to Lewis teaches a wax composition containing a graft polymer of polyethylene used to coat paperboard. The preferred coating is composed of 96-99.1% wax such as paraffin or microcrystalline waxes and 0.1-4% of a polyethylene graft copolymer where the polyethylene contains 0.1-5% by weight of maleic anhydride grafted to the polyethylene chain. This coating can be applied as a hot melt or by a solvent system. It gives superior wet strength to paperboard.
U.S. Pat. No. 3,287,149 to Dooley et al. teaches repulpable printed paper having a water-resistant coating formed from a copolymer that is the copolymerization product of (a) styrene with (b) a half-ester of an alcohol having 1 to 8 carbon atoms with an unsaturated dicarboxylic acid such as fumaric or maleic acid. These compositions are applied by dissolving the polymers in water. They are removed by exposing the coated paper to a 2% aqueous sodium hydroxide solution of the type commonly used in commercial paper de-inking processes. However, the Dooley et al. Patent compositions are applied as aqueous solutions and thus are not suitable for application by conventional cascade or curtain coating equipment.
U.S. Pat. No. 3,262,838 to Vieth et al. is similar to the Dooley '149 Patent above. However, Vieth et al. teach a repulpable paper made from a coating composition applied from a solvent solution that comprises 15-30% of an acrylic acid/acrylic ester polymer and a thermosetting aminoplast resin. The entire water-resistant coating composition is composed of these two ingredients. Paper coated with the composition is said to be repulpable in a dilute alkaline aqueous solution. However, the compositions are applied from solvent solutions, not from conventional cascade or curtain coating equipment.
U.S. Pat. No. 2,391,621 to Powell, III, et al. teaches a method of coating paper to increase its moisture resistance using a coating composition based on a polymer containing three different components: maleic anhydride, vinyl halides and vinyl esters of lower fatty acids. The coating may contain a wax in an amount of from about 1-50% of the polymer. Nothing is taught about the repulpability of such a composition which is applied from a solvent solution or from an aqueous emulsion.
As noted in the Michelman et al. Article above, water-based coating compositions have also been pursued for making paper and paper-based containers water-resistant. These coatings have been described as leaving the paper products repulpable.
U.S. Pat. No. 4,117,199 to Gotoh et al. teaches a coated paper made by coating a paper substrate with an aqueous emulsion containing a synthetic rubber latex and a wax emulsion in an amount of 5 to 200 parts by weight of the solid wax per 100 parts by weight of the solid synthetic rubber in the latex. The coated paper is then dried at a temperature that is the same as or higher than the melting point of the wax. The resulting coated paper is said to be highly repulpable. The synthetic rubber latex is made from copolymers of butadiene with at least one ethylenically unsaturated monomer such as methyl methacrylate, styrene and acrylonitrile. The synthetic rubber may further contain at least one ethylenically unsaturated carboxylic acid such as acrylic acid, methacrylic acid, crotonic acid and maleic acid. The Gotoh et al. compositions are aqueous and are not applied from conventional cascade or curtain coating equipment.
U.S. Pat. No. 2,576,914 to Barrett teaches coated papers made by applying a dispersion or suspension of a suitable pigment along with a long chain unsaturated fatty acid and a copolymer that is an alkyl half-ester of maleic anhydride copolymerized with a polymerizable vinyl compound. These coating compositions are preferably applied as aqueous liquids. A wax can be present in small proportions as an adjuvant.
British Pat. No. 1,593,331 to Vase teaches a method for treating paper and paperboard to make them water and vapor resistant by coating them with an unpigmented aqueous latex coating composition. The latex coating composition is an acrylic polymer and a metal stearate or wax where the wax is at least 20% by weight of the total acrylic polymer and metal stearate present. The metal stearate is preferably calcium stearate. After application, the coating is dried onto the paper. It contains 50-80% acrylic polymer.
Other examples of aqueous emulsions and emulsifiable compositions for coating paper and the like are found in U.S. Pat. Nos. 3,020,178 to Sweeney et al. (dispersions of water insoluble linear polymers of esters of acrylic acid or methacrylic acid which may further contain adhesion-promoting agents such as carboxylic acids in the form of free acids or salts); U.S. Pat. No. 3,392,131 to Miles et al. (salts of ethylene-crotonic acid polymers as emulsifying agents for paraffin waxes and asphalt); and U.S. Pat. No. 3,520,842 to Crean (blends of petroleum wax, a polymeric olefin material and a fatty acid are added as a blend to water containing an amine soap-forming agent such as an alkanolamine, agitating the mixture and subjecting it to homogenization to form an aqueous emulsion coating composition).
Japanese Kokoku Pat. No. HEI 2[1990]-1671 to Yamazaki et al. teaches water vapor-resistant corrugated board made by coating the board with a mixture of an acrylic emulsion and a wax emulsion and drying the coating. The objective is to obtain recyclable paperboard. The acrylic emulsion is said to be an "acryl-styrene" emulsion in an amount of from 90:50 to a wax emulsion in an amount of 10-50. Due to the water present, the coatings must be dried using hot-air heating, infrared heating or microwave heating.
Japanese Kokai Patent Application No. SHO 56[1981]-91097 to Kano et al. teaches a method for the manufacture of paper for fruit bags. The paper is treated with an aqueous emulsion consisting of 81-93% of petroleum wax, 5-15% of fatty acids such as stearic acid and 2-4% of a resin such as a melamine-formaldehyde resin or a methyl methacrylate resin. These coatings are said to make the paper water-repellent for a long period. The resins can be selected from one or more of styrene, methylmethacrylate, and methacrylic acid resins.
Japanese Kokai Patent Application No. SHO 49[1974]-85305 to Fujiya et al. teaches a method for the manufacture of waterproof paper that is redispersible with warm water. It is made by soaking the paper with a polyethylene emulsion or with blends of aluminum salts and waxes such as paraffin or zirconium salts and wax. The coating material was only identified by the manufacturer's name for the materials used.
U.S. Pat. No. 4,748,196 to Kuroda et al. teaches a water repellent composition which is an aqueous emulsion prepared by emulsifying in water (A) a wax having a melting point from 40.degree. C. to 90.degree. C. and (B) a 1-alkene/maleic anhydride polymer, or a partially esterified product thereof, in the presence of (C) a water soluble alkaline compound and/or a nonionic emulsifier. The emulsions are used to impart water repellency to gypsum, cement, paper, wooden boards or fibers. Nothing is suggested regarding repulpability of articles treated with such emulsions.
U.S. Pat. No. 3,312,564 to Barbour teaches transfer sheets that are a base sheet coated with an aqueous emulsion of a hard wax, a non-film forming resin, a fatty acid and a base followed by hardening the coating onto the base sheet. The coatings are heated to transfer them permanently to a substrate where the heat fixes the transfer to the substrate and makes it smudge proof. Nothing is taught about the use of such coatings for making paper substrates water-resistant and later repulpable.
U.S. Pat. No. 2,275,659 to Steinle et al. teaches a water removable wax coating that is placed on the skin or peel of a fruit or vegetable. The coating is an aqueous coating containing both a wax and a substance that will swell upon contacting a volume of water thereby causing the protective coating to loosen itself from the skin or peel. Examples of such swellable materials are bentonite, gelatin, Irish moss and gums such as karaya, tragacanth and arabic. Since these compositions are always sensitive to water, they could not be used to render paper or paperboard products water-resistant.
In the area of coatings for substrates such as floors, U.S. Pat. No. 3,320,196 to Rogers teaches a coating composition containing a polyligand emulsion polymer that is (a) a terpolymer of a methacrylate monomer, an unsaturated monomer having a carboxylic acid group and an unsaturated monomer that is free of methyl groups on the alpha carbon atom, (b) a resin cut of a low molecular weight alkali soluble resin which can be a maleic anhydride or acrylic acid resin, (c) an aqueous base, and (d) a zirconium derivative that is used to reversibly cross-link the polymers. Waxes and fatty acid soaps may also be optionally included. The coatings formed can be removed by exposing them to aqueous alkaline solutions. The coatings function as floor polishes that are strippable using aqueous alkaline solutions.
U.S. Pat. No. 4,013,607 to Dwyer et al. teaches self-stripping coating compositions similar to those taught by Rogers which may include waxes, but which require a pH of greater than 9 and do not require the use of zirconium-derivatives to achieve self-stripping properties. These compositions are said to be useful for coating rigid surfaces such as for use as self-strippable floor coatings although they can also be used to impregnate textiles, leather, paper, or other porous or fibrous materials.
Thus, although a number of different types of coatings have been proposed, there is still a need for a hot melt coating composition for use in rendering fibrous materials such as paper and paper-based materials water-repellent using conventional curtain coating equipment, but which permits the coated paper or paper-based product to be recycled using conventional repulping equipment and techniques.